If it has gone to a collection agency, you must have gotten several previous IRS notices that you did not respond to. It may now be too late to challenge the amount that they say you owe. If you want to try to do anything about it, you will need professional help.
It's very unlikely that TurboTax miscalculated, but at this point it doesn't matter since the IRS has already assessed the additional tax and it has gone to collections.
